The next-generation optical network desires an OSNR measuring device that monitors an OSNR of an optical signal in operation without giving any effect to a main signal, for fault detection, early recovery from the fault, and realization of a network control technology for increasing the transmission capacity.
The OSNR measuring device receives an optical signal by using, for example, a photodiode, and measures an OSNR of the optical signal on the basis of the signal intensity of a Direct-Current component (DC component) in the received optical signal and the noise intensity of an Alternating-Current component (AC component) having passed through an optical filter. The OSNR measuring device can measure an OSNR of the optical signal in a simple configuration and at low cost.    Patent Literature 1: US Patent Application Publication No. 2015-0155935    Patent Literature 2: Japanese Laid-open Patent Publication No. 2015-106905
Recently, a super channel transmission technology for 400 G/1 T transmission uses an optical filter whose bandwidth is approximately 0.1 nm in consideration of influence of crosstalk from adjacent subcarriers because a signal band of subcarrier signals that constitute a super channel is 32 GHz and a bandwidth thereof is approximately 0.26 nm. In a super channel transmission technology, a frequency bandwidth of an optical filter is narrowed.
However, in the OSNR measuring device, deterioration in OSNR monitoring accuracy is more significant as a frequency bandwidth of an optical filter becomes narrower. For example, the amount of amplifier noise included in the noise intensity decreases relatively to an optical signal. As a result, in such a case that the bandwidth is 0.2 nm or less, the monitoring accuracy of an OSNR deteriorates as illustrated in FIG. 10.
Therefore, in order to increase the monitoring accuracy of an OSNR, it is considered to execute an averaging process and increase the monitoring accuracy of an OSNR by increasing the averaging count in the averaging process. However, because a time until the monitoring accuracy is improved is longer as the averaging count increases, measuring time of an OSNR increases.